In the manufacture of lamp assemblies, a light emitting glass lamp such as a fluorescent glass tube or an incandescent glass bulb usually containing wires or filaments is affixed to a metal end cap or base by a thermosetting cement. In order to affix the glass lamp to the metal base, the base is lined with a generous amount of cement, and is then warmed to soften the cement. The base is then placed on one the end of the glass lamp and the cement is heated and cured while the base and glass lamp are securely held together.
Typical cement formulations comprise mixtures of synthetic phenolic resins like novolak and natural resins like shellac and rosin. These binders are used along with fillers, hardeners, solvents, and other reactive components like aldehydes, ammonia, and metal hydroxides for in-situ condensation polymerization. Other cements may contain epoxides, polyesters, alkyds, acrylics, or silicone resins.
These cements, however, have several drawbacks. First, the cements have short shelf lives because of the need for solvents like trioxane or hexamethylenetetramine and other reactive components like aldehydes, ammonia, or metal hydroxides. Second, the use of these organic solvents in the high temperature curing process for these cements creates both health risks and environmental concerns. Third, accurate application of these cements is difficult, and as a result, a relatively large amount of cement must be used to adhere a glass lamp to a metal base. Fourth, because it is very difficult to apply these cements evenly to lamp bases either an insufficient amount of cement is used resulting in inadequate bonding between the glass lamp and the base, or too much cement is used resulting in lamp breakage or impairment of the function of the lamp assembly. Finally, these cements often cannot withstand the high temperatures, which are required during the lamp assembly manufacturing process.
Various attempts to improve the methods for adhering glass lamps in metal bases have been made and some of the compositions and methods for this purpose are described in the prior art. For example, British Patent Specification No. 1,139,266 discloses the use of an insulating foamable hollow cylinder comprising a novolak, hexamethylenetetramine, and dolomite resin to fill the free space in an electric incandescent lamp base in order to prevent arc formation between the current supply conductors or between such conductors and the base. The hollow cylinder is placed in the base, and the incandescent lamp or bulb is then affixed to the base with a conventional thermosetting cement. During the cementing process, the hollow cylinder will foam and fill the free space in the base and prevent formation of arcing between the current supply conductors.
German Patent Application No. 1,958,307 discloses the use of a foamable putty to secure a gas-filled incandescent lamp to a base. The putty or cement is a conventional heat curable putty with a heat foaming additive comprising phenolic resin, hexamethylenetetramine, marble flour, and talc powder. The putty is spread in the upper edge of the lamp base, the lamp is placed in the base, and the base is heated so that the putty foams and fastens the glass lamp to the base.
Japanese Patent Application (Kokai) Nos. 56-9931 and 56-9932 disclose the use of dimer acid based polyamide and polyacrylamide (and its copolymer polyvinyl) based resin cements for use in securing glass tubes or bulbs to bases. While polyamide and polyacrylamide based cements may be more environmentally friendly, they suffer from other disadvantages. These cements are quite expensive, and on prolonged exposure to heat become brittle due to oxidative degradation. Finally, polyamides are also very tacky, and as a result, are difficult to compound into usable compositions.
Japanese Patent Application (Kokai) No. 59-121764 discloses the use of a foamable tape comprising an epoxy or polyester resin and a blowing agent to secure a fluorescent tube or incandescent bulb to a metal base. In order to secure the glass tube or bulb to the base, the foamable tape is wound around the end of the glass bulb, the bulb is placed in a base, and the resulting assembly is heated to 120° C. to foam or expand the adhesive tape. However, it is very difficult to manufacture a tape out of epoxy or polyester thermosetting resins like these, and still maintain control over the tape's dimensions.
Patent Application WO 98/28359 discloses the use of an elastic foamable sealing material, which can be used in lamp housings. The foamable sealing material comprises modified silane polymers, fillers, silica, softeners, and organo-functional low-molecular weight silanes.
U.S. Pat. Nos. 4,988,912 and 4,888,519 disclose using thermoplastic resin rings comprising polyetherimide and polyethersulphone respectively for adhering a lamp vessel, shown as a bulb, to a base. In use, the thermoplastic ring is heated to a temperature of 150-200° C. and placed on the end of a heated lamp vessel. The lamp vessel is then mated with the base, and the base is heated to a temperature of 400-450° C. to melt the ring thereby adhering the lamp vessel and base. The disclosed thermoplastic resin rings are not foamable in nature and suffer from the disadvantage that high heat is required to sufficiently melt the rings to secure the lamp vessel to the base.
In addition, numerous patents disclose the use of EVA based foamable materials for various unrelated applications. For example, U.S. Pat. No. 6,114,004 to Cydzik et al. and U.S. Pat. Nos. 6,107,574, 5,979,902, and 5,931,434 to Chang et al., which are all owned by the assignee of the present application, disclose the use of sealing articles comprising a driver and a sealer in which the driver and sealer are EVA based foamable compositions. The sealing articles are useful in sealing cavities in automobile frame channels and substrates such as electrical conductors and optical fibers. While the foamable compositions of Chang et al. and Cydzik et al. may be able to withstand elevated temperatures for relatively short periods of time, the foam structure in these compositions would collapse on prolonged exposure to elevated temperatures for an extended period of time, such as 140° C. for 2000 hours. Also, the useful time-temperature window for installation of these compositions is very narrow because exposure to high temperatures results in grossly non-uniform cell structure. Furthermore, these foamable compositions would not be able to withstand exposure to commonly encountered hot and humid storage conditions, such as 45° C. at 80% relative humidity for one week.
U.S. Pat. No. 4,456,784 discloses the use of a foamable cylindrical barrier for use in an electrical conduit to prevent the flow of vapor through the conduit. The disclosed foamable cylindrical barrier may comprise an EVA copolymer and a sufficient amount of dicumyl peroxide to cause the barrier to foam.
PCT Patent Application No. WO 97/47681 discloses a reversibly deformable pressure sensitive adhesive foam comprising an EVA copolymer and an expandable particulate material comprising a polymeric shell and a volatizable fluid or gas core. The disclosed pressure sensitive adhesive foam is useful for decorative trim pieces on automotive bodies, appliances, home and office furnishings and equipment. The disclosed foam would not be suitable for use in securing glass lamps to bases, as the underlying technology and normal applications for such foams are entirely different than that of the present invention. As a result, the pressure sensitive adhesive foam is not designed for and would not be capable of withstanding prolonged exposure to elevated temperatures, i.e., 140° C. for 2000 hours, as required by IEC standards.
While the above foamable compositions and methods for securing glass lamps in bases may be suitable for their intended purpose, it is believed that there is demand in the industry for an improved composition for securing light emitting glass lamps in bases, especially one that can be easily and inexpensively prepared, is environmentally friendly, and which can also withstand the elevated temperatures lamp assemblies are required to endure for extended periods of time. It is further believed that there is a demand for an improved composition for securing light emitting glass lamps in bases which is easy to handle and can be used in existing manufacturing process for lamp assemblies.